1. Field of the Invention
The present invention relates to a self-sealing tyre for vehicle wheels.
More particularly, the present invention relates to a self-sealing tubeless tyre for vehicle wheels, to a process for manufacturing said tyre, and to a sealing material used therein.
2. Description of the Related Art
In the industrial field of tyres for vehicle wheels, various attempts have been made to produce self-sealing tyres, in other words tyres which can retard or prevent the loss of air and their consequent deflation following a puncture caused by a sharp object (a nail, for example).
In the known art, descriptions have been given of self-sealing tyres provided with at least one layer comprising a polymeric material which can adhere to the object causing the puncture and can also flow into the puncture site when said object is removed, thus sealing the puncture and preventing the outflow of air from the tyre.
For example, U.S. Pat. No. 3,981,342 describes a self-sealing tubeless tyre provided with a layer including a composition comprising a mixture of a low molecular weight liquid elastomer and a high molecular weight solid elastomer, and a quantity of a cross-linking agent sufficient to give a partial cross-linking of said mixture, said liquid elastomer being present in a greater quantity than said solid elastomer. Examples of low molecular weight liquid elastomers which can be used according to the invention are: liquid cis-polyisoprene (for example, heat-depolymerized natural rubber, or low molecular weight depolymerized cis-polyisoprene); liquid polybutadiene; liquid polybutene; liquid EPDM; liquid butyl rubber. Examples of high molecular weight elastomers which can be used according to the invention are highly unsaturated rubbers such as, for example, isoprene homopolymers (particularly natural or synthetic cis-polyisoprene); polybutadiene (particularly with a high cis content); polychloroprene; copolymers containing a major proportion of conjugated dienes (butadiene, for example) and a minor proportion of copolymerizable monomers containing monoethylenically unsaturated monomers (styrene or acrylonitrile, for example). Alternatively, it is possible to use elastomers with a low degree of unsaturation such as butyl rubbers (isoolefin copolymers, for example isobutylene, with small quantities of dienes, for example isoprene), or EPDM-type rubbers (copolymers of at least two different monoolefins such as ethylene and propylene with a small quantity of a non-conjugated diene such as dicyclopentadiene, 1,4-hexadiene, 5-ethylidene-2-norbornene). Finally, it is also possible to use saturated elastomers such as EPM or ethylene-vinyl acetate copolymers. Examples of cross-linking agents which can be used according to the invention are: sulphur or sulphur donors; quinones; organic peroxides; polyisocyanates; and tetrahydrocarbyl titanate esters. Said layer would be able to impart self-sealing properties to the tyre.
U.S. Pat. No. 4,140,167 describes a sealing material usable, in particular, in the field of tyres. In the tyre, said sealing material can consist of a single layer, or a composite laminar structure. Preferably, said composite laminar structure comprises at least five layers of elastomeric material of which: two outer layers comprising an elastomeric material and an agent for retarding cross-linking by radiation, two inner layers comprising an elastomeric material and an agent for promoting cross-linking by radiation, and one layer, located between said two inner layers, comprising said sealing material which is at least partially degradable by radiation, or by heat in the presence of a peroxide. Examples of materials which are degraded by radiation, or by heat in the presence of a peroxide, useful according to the invention are polyisobutylene and its copolymers, or polyethylene oxide. Said sealing material would be able to impart self-sealing properties to the tyre.
U.S. Pat. No. 4,228,839 describes a self-sealing tyre provided with a layer comprising a mixture of a radiation-degradable polymeric material and a polymeric material which can be cross-linked by radiation and/or by heat. The degradable polymeric material can be selected from: polyisobutylene; copolymers of isobutylene with: conjugated olefins such as 1,3-butadiene, isoprene, 2,3-methylbutadiene, or with vinyl aromatic monomers such as styrene and alpha-methyl styrene, or with vinyl polar monomers such as acrylonitrile, methylvinyl acrylate, methylvinyl lactone. The cross-linkable polymeric material can be selected from natural rubber; styrene/butadiene copolymers; polybutadiene; polyisoprene; block copolymers, for example styrene/butadiene/styrene; halogenated butyl rubbers, for example chlorobutyl rubber; ethylene-propylene terpolymers; butadiene-acrylonitrile copolymers; neoprene. Said layer would be able to impart self-sealing properties to the tyre.
Patent application EP 127 998 describes a tubeless tyre in which the inner liner is a laminar structure formed by two outer layers comprising a halogenated butyl rubber and an inner layer comprising a butyl rubber and a peroxide. After vulcanization, the butyl rubber of said inner layer would be degraded, because of the presence of the peroxide, thus producing a polymeric material having a Mooney viscosity (ML 1+4) in the range from 2 to 12. Said polymeric material would be able to impart self-sealing properties to the tyre.
U.S. Pat. No. 4,895,610 describes a self-sealing tyre provided with a layer comprising a butyl rubber and a peroxide. Preferably, said layer comprises: 100 parts of butyl rubber; about 10 to 40 parts of carbon black; about 5 to 35 parts of polyisobutylene; about 5 to 35 parts of an oil; about 0 to 1 parts of sulphur; and about 1 to 8 parts of a peroxide. During the vulcanization of the tyre, the butyl rubber, in the presence of the peroxide, would be capable of being partially cross-linked, thus providing dimensional stability to said layer, and also of being partially degraded, thus providing a low-viscosity tacky material capable of imparting self-sealing properties to said tyre.